Should a Sill Plate Be Mounted Flush to the Outside of a Foundation Wall?

Sill Plate Properties

• The sill plate is a few inches narrower than the width of the foundation wall. For example, if you have a standard 8-inch basement wall, you would use 2-by-6-inch lumber to construct the sill plate. The actual dimensions of a 2-by-6 are 1 1/2 inches by 5 1/2 inches. Sill plates must be constructed from treated lumber. You also need to install a moisture barrier between the sill plate and the foundation, which may be brushed-on sealer or polyethylene film.

Squaring the Plate

• When you square the sill plate, you ensure that the angles of the plate are exact. Because a foundation wall may have some measurement discrepancies, you cannot use it as a guide for squaring the sill plate. Instead, you measure from the outside corners of the foundation to a point that is the same distance as your sill plate's width. When using a 2-by-6 plate, you would measure 5 1/2 inches in from the outside corners and make precise marks on the top of the foundation wall. By snapping a chalk line between these marks, you create a straight guideline to install the sill plate.

Anchor Bolts

• The sill plate attaches to the foundation with anchor bolts, which already extend from the top of the concrete wall. These bolts are not even, however, since the foundation contractor inserted them by hand while the concrete was wet. To attach the sill plate, you have to drill holes that correspond to the locations of the bolts.

Drilling Plate Holes

• While holding the sill plate along the top of the foundation wall, next to the bolts, you make a mark on the edge of the plate by each bolt. Measure the distance from the chalk line to the center of each bolt, and use that measurement to determine where to drill each bolt hole in the plate.

Attaching the Sill Plate

• Once you drill the holes in the sill plate, you can fit the plate over the anchor bolts, and it should be flush with the chalk line. It should also be nearly flush with the outside foundation wall, but you may notice that the foundation extends a little here or there, which isn’t important since your sill plate is square.